1. Field of the Invention
The present invention relates to a method and apparatus for detecting heartbeats, and more particularly, to means for preventing pacemaker artifacts from being erroneously detected as QRS complexes.
2. Description of the Prior Art
Establishing the condition of the heart of a patient is typically accomplished by monitoring its electrical activity. Patient monitoring systems develop and analyze ECG signals which are responsive to the electrical activity of the heart. One type of analysis carried out by a patient monitoring system is to detect the QRS complex portions of the ECG signals, whose output is used to measure heartrate. QRS detectors typically comprise an amplitude or slew-rate detector since the QRS complex is normally both the fastest rising and largest amplitude (typically 0.25-2 millivolts) component of a normal ECG signal. However, this is not always true. For example, in the case of a patient having a pacemaker, the pacer signals (artifacts including an impulse of typically 2-700 millivolts) may be either erroneously detected as a QRS complex or may so disrupt the QRS detector that it will not be able to properly function. Since persons with pacemakers are a class of people for which patient monitoring is extremely important, it is critical that QRS complexes, as well as other portions of the ECG signals are properly analyzed by the patient monitoring system and are not disrupted or " fooled" by the pacer artifacts so that accurate heartrates can be measured.
Accordingly, patient monitors typically contain circuitry for detecting pacemaker artifacts. The output of this detector is used by the QRS detector to identify what portion of the incoming ECG data contains the pacemaker artifact. A common technique used in QRS detection is to remove or blank the ECG data corresponding to the pacer artifact before it is processed by the QRS detector. An example of this technique is shown in the idealized ECG signal illustrated in FIGS 1a and 1b.
FIGS. 1c and d illustrate examples of an ECG signal before and after pacer impulse blanking, respectively, which signal has a relatively large repolarization tail and no subsequent heart activity. Unfortunately, the above described technique does not work well during this situation because that portion of the artifact which remains after blanking may be large enough to be detected by a typical QRS detector. Increasing the amount of ECG data being blanked is not a practical solution because if a QRS complex occurred during the time that the ECG data is blanked, it would be missed by the detector.
In another technique, the QRS detector algorithm reduces the sensitivity of the QRS detector during the time immediately following detection of a pacer artifact. Although this may improve the performance of the detector in the presence of large pacer artifacts, it may also cause low-amplitude QRS complexes to be missed by the detector.
Accordingly, it is an object of the present invention to provide a method and apparatus which will allow for the accurate detection of QRS complexes in the presence of pacer artifacts.